Optical information recording media such as a magneto-optical recording medium and a phase-change recording medium are known for making it possible to record large amounts of data, and making it possible to reproduce and rewrite data at high-speeds. To record data, these optical information recording media take advantage of the differences in the optical characteristics of recording materials caused by localized irradiation by a laser beam. For example, a magneto-optical recording medium records data by taking advantage of the differences in the rotational angle of the plane of polarization of the reflected light that result from different states of magnetization. Also, a phase-change recording medium records data by taking advantage of the fact that the amount of light reflected when a laser of a predetermined wavelength is irradiated onto a material in a crystallized state and an amorphous state is different. And, because it is possible to erase records and rewrite records simultaneously by modulating the output power of the laser beam, the phase-change recording medium has the advantage that data signals can be rewritten at high speed.
Examples of the structure of phase-change recording medium are shown in FIGS. 3A and 3B. The recording medium shown in FIG. 3A is structured with a substrate 21, a first protective layer 22, a recording layer 23, and a second protective layer 24 layered in that order from the side on which the laser beam is irradiated.
Polycarbonate, polymethyl methacrylate (hereafter, PMMA) or other such resins, or glass or the like, is used for the substrate 21, and a guiding groove is generally provided to guide the laser beam.
The recording layer 23 is made from materials that exhibit reversible changes between states with different optical characteristics. In the case of rewritable-type phase-change recording medium, known examples of these include: Te—Sb—Ge, Te—Sn—Ge, Te—Sb—Sn—Ge, Te—Sb—Ge—Se, Te—Sn—Ge—Au, Ag—In—Sb—Te, In—Sb—Se, and In—Te—Se.
Materials for the first and second protective layers 22 and 24 include: a sulfide such as ZnS; an oxide such as SiO2, Ir2O5, or A2O3; a nitride such as GeN, Si3N4, or Al3N4; or an oxynitride such as GeON, SiON, or AlON; a dielectric such as a carbide or a fluoride; or an appropriate mixture of materials such as these. However, the materials that are mainly used are mixtures of ZnS and SiO2.
Furthermore, a phase-change recording medium is shown in FIG. 3B in which a reflective layer 25 is further provided. The reflective layer 25 is ordinarily made of a metal such as Au, Al, Ag, or Cr, or of an alloy including at least one of Au, Al, Ag and Cr as the primary metal. The reflective layer 25 is provided for purposes such as effective heat dissipation and light absorption of the recording layer 23.
There are three major reasons for the above-described structure in which the recording layer 23 is positioned between the two protective layers 22 and 24, or between the protective layer 22 and the protective layer 24 with the reflective layer 25. The first reason is that the recording layer 23 is made amorphous by melting and quenching when recording, so this structure is provided to maintain the form of the recording layer 23 and prevent mechanical deformation. The second reason is that, by raising the laser absorption efficiency of the recording layer 23, the change of the reflectance between the amorphous state and the crystallized state can be made larger, thus providing the optical effect that the amount of reproduction signals can be increased. The third reason is in order to control the thermal conditions necessary for making the recording layer 23 amorphous and crystallized. In particular, a known structure (quenching structure) for obtaining the necessary quenching conditions for making the material amorphous is one in which the protective layer 24 is made thin so that the heat of the recording layer 23 easily can escape through to the reflective layer 25.
Furthermore, although omitted from the drawings, structures are generally used in which an overcoat layer is provided over the second protective layer 24 or in which a dummy substrate is laminated using a UV-curing resin as an adhesive for the purpose of preventing oxidation and adherence of dust or the like on the recording medium.
Among various methods for forming each of the layers in a phase-change recording medium such as these, the method generally used is the so-called single-wafer film formation method in which independent film formation chambers are used successively to form each layer made of different materials.
Because the first protective layer 22 is designed to have the same or a greater thickness compared to the other layers, it requires more time for film formation than the other layers. Consequently, in the production processes of the phase-change recording medium, the film formation process for the thick film of the first protective layer 22 limits the overall production speed. Thus an issue for improving productivity is how to reduce the film formation time of the first protective layer 22.
Increasing the rate of film formation by way of the film formation conditions is a conceivable method for reducing the film formation time. For example, in the case of using a sputtering film formation device for film formation, possible methods include increasing the sputter power or reducing the distance between the target and the substrate to improve adherence efficiency, as well as increasing the diameter of the target. However, because there are large temperature rises on the film adhering surface of the sample with these methods, heat-caused deformation, that is, tilt change, can occur when the substrate is made of resins such as polycarbonate. These method are therefore not preferable. For example, the glass transition temperature of polycarbonate is 150° C., and because it is necessary that the substrate temperature not be raised above the glass transition temperature in order to prevent deformation, it is difficult to use such methods.